[Elecraft] Filter Selection Approach

[Bill W4ZV]

See K8ZOA's plot for the "500 Hz 5-pole CW/Data Filter" section about half way down this page:

http://tinyurl.com/dho6j6


While looking at those plots, note that the slope of all the skirts is about the same once you're inside the roofing filter (i.e. beginning at 5-600 Hz).  This is because the slope is determined mainly by the DSP.  Lyle once told me exactly what that slope was but I've forgotten it.  The reason apparent DSP shape factors increase at narrow BWs is because the DSP skirt slope is a constant in dB/Hz, and independent of DSP BW.  Of course a wider shape factor is good because you don't want extremely narrow filters to ring, which could happen if you made a 100 Hz filter with an extremely narrow shape factor.  

BTW, the FIR/IIR options for the 100/50 DSP filters is another area K8ZOA examined below:

http://tinyurl.com/d5skb8

73,  Bill


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